The human immune response genes are located in the D region of the major histocompatibility complex. They encode the Alpha and Beta chains of the HLA-DR, -DQ and -DP class II antigens. The aim of this project is to define by a molecular approach how many functional class II genes exist and to express individual class II antigens in human cell lines by DNA-mediated gene transfer in order to analyze the interaction between class II antigens and T lymphocytes. A cDNA library in a eukaryotic expression vector was constructed from mRNA of a human B-cell line having a single HLA haplotype. Expressible clones for the Alpha and the Beta chains of DP, DQ and DR antigens were isolated. In addition, a clone for a new Beta chain, designated DO, was isolated and sequenced. The DOBeta gene maps in the D region but has evolved independently from the DPBeta, DQBeta and DRBeta genes and is subject to different regulatory mechanisms. The atypical evolution and expression of DOBeta suggest that it may be part of a new class II antigen with a distinct function. Human fibroblast lines transfected with the DR1 Alpha and Beta genes have been obtained which express DR antigens at the cell surface. These DR1 antigens are functional; the transfected cells are recognized by DR1-restricted cytotoxic T cell clones specific for measles virus when the cells are infected with measles virus. This system can be used to define the elements important for antigen-recognition by T lymphocytes.